Climate change could bring out the worst in poison ivy. New research demonstrates that adding carbon dioxide to the air makes poison ivy grow faster and produce a stronger form of “poison,” suggesting that these woody vines will be more plentiful and more potent by the middle of the next century.

Worst of all, this super poison ivy could threaten forests around the world, say Jacqueline Mohan of Duke University in Durham, North Carolina, and six coauthors in Proceedings of the National Academy of Sciences. Woody vines are already increasingly abundant globally, strangling young trees and thus inhibiting forest growth and regeneration—and increased carbon dioxide is a likely culprit. Although excess carbon dioxide makes both vines and trees grow faster, vines get more of a boost because they grow relatively more leaves, whereas trees grow relatively more support tissue. And having more leaves accelerates vine growth even further because more leaves mean more photosynthesis.

To see how increased carbon dioxide affects poison ivy, the researchers compared the vines’ growth and potency in three forest plots with current CO2 levels and in three plots with the levels predicted for the year 2050 (about 370 vs. 570 microliters per liter of air, respectively). The experiment was conducted at Duke’s Free-Air CO2 Enrichment (FACE) site, where carbon dioxide levels are controlled in intact forest plots. Located in a loblolly pine (Pinus taeda) plantation, the site has 30-meter-diameter plots encircled by 32 vertical pipes that blow carbon dioxide toward the center, controlling levels of the gas to a height of 16 meters.

Over the course of the six-year study, the researchers demonstrated that adding carbon dioxide boosted the growth of poison ivy by more than 60 percent. The average biomass per poison ivy plant was about eight grams at the higher carbon dioxide level, compared to five grams at the current level. Moreover, more carbon dioxide increased the vines’ photosynthesis by more than 75 percent.

More carbon dioxide also made poison ivy more “poisonous.” The vines’ allergen, urushiol, has two forms:saturated and unsaturated. Unsaturated urushiol causes more severe reactions. At the higher carbon dioxide level, poison ivy had 50 percent more unsaturated urushiol and 60 percent less of the less allergenic, saturated urushiol.

These findings are likely to apply to other urushiol-producing plants such as poison oak (T. diversilobum) and poison sumac (T. vernix). “Our results indicate that Toxicodendron taxa will become more abundant and more ‘toxic’ in the future, potentially affecting global forest dynamics and human health,” say the researchers.